The present invention is related to fluidic demand apparatus, in general, and more particularly, to fluidic demand apparatus employing a microvalve or micro electro-mechanical system (MEMS) flow sensor, and the microvalve or MEMS flow sensor itself.
An example of a fluidic demand apparatus includes an Oxygen conserver which is shown by way of example in the fluidic schematic diagram of FIG. 1. An Oxygen conserver controls the flow of Oxygen gas from a source to a patient on demand, i.e. when a patient inhales. Referring to FIG. 1, in fluidic demand apparatus, the fluid, like Oxygen gas, for example, is generally provided from a high pressure source, such as a storage tank 10. From the tank 10, the fluid is usually regulated by a regulator 12. A pressure gauge 14 may be provided at the tank 10 as an indication of the fluid remaining in the tank 10. In the present example, the fluid in the tank 10 is at a pressure of 2,000 pounds per square inch (psi) and the regulator 12 reduces the pressure to approximately 40 psi.
The fluid may exit from the regulator 12 at a pressure of approximately 40 psi through two tubes or passageways 16 and 18. The tube 16 may be coupled to a delivery tank 40 which is coupled through a tube 22 to an input of a shuttle valve 24. A variable flow restrictor 25 may be disposed at the tube 16. An output of the shuttle valve 24 is coupled through a tube 26 to a passageway 28 leading to the patient. Within the valve 24 is a piston 30 which is movable from a bottom or closed position to a top or open position (see dashed lines). The tube 18 may be coupled to a tee connection 32 which may be coupled to the top of the valve 24 through a tube 34 and to a bottom of a diaphragm container 38 through a tube 36. Fixed fluid flow restrictors 40 and 42 may be disposed at the tubes 18 and 36, respectively. Another tube 44 may couple the bottom of container 38 to the atmosphere through a variable restrictor 46. Yet another tube 48 couples a top of container 38 to the patient's tube 28 through a check valve 50. A diaphragm 52 within container 38 may be in a spring loaded position (solid line) to close off a passage between tubes 36 and 44.
In operation, when the patient starts to inhale fluid through tube 28, fluid is conducted through the check valve 50 in tube 48 which creates a pressure differential across the diaphragm 52 in container 38. When the differential pressure overcomes the spring bias force, the diaphragm 52 is forced upwards (see dotted line position) which permits fluid to flow from the regulator 12 through tubes 18 and 36, through an open passageway in container 38 and through tube 44 exiting to the atmosphere. Thus, the fluidic pressure holding piston 30 in valve 24 in the closed position is relieved allowing piston 30 to rise to the open position (dotted line). In this position, fluid flows from the delivery tank 20 through tubes 22, 26 and 28 to the patient. The apparatus will remain in this state while the patient is inhaling.
When the patient stops inhaling, the spring bias force on diaphragm 52 forces it downward to block the fluid passageway between tubes 36 and 44. In this state, fluidic pressure builds up in tube 34 to force the piston 30 to the closed position (solid line), thereby closing off the fluid flow between tubes 22 and 26 and to the patient via tube 28. The foregoing described operation will repeat itself upon demand. In the present example, this demand results from commencement of inhalation of the patient. Note that the demand should be sufficient enough to overcome the spring bias of the diaphragm 52 in container 38. Otherwise, no fluid will flow to the demanding entity. The fluid flow in the present example is limited by the various restrictors in the tubes. In some apparatus, the valve 24, diaphragm container 38 and restrictors 40, 42 and 46 may be integrated in a common mechanical unit.
The foregoing described mechanical fluidic demand apparatus is adequate for controlled delivery of fluid to a demanding entity; however, it has a number of drawbacks. For example, such apparatus is comprised of many individual fluidic components which are complex and expensive to assemble. The overall manufacture of such apparatus generally involves special tooling, and set-up and quality assurance procedures. In addition, the mechanical fluidic apparatus is difficult to service in the field leading to reliability and cost issues. Generally, field service of the apparatus involves replacement of parts. Also, from a clinical perspective, the response to patient inhalation is not considered sensitive enough for triggering fluid flow, i.e. the patient has to draw harder.
The present invention overcomes these drawbacks of the current fluidic demand apparatus by replacing the mechanically active parts with miniature, low power electrically operative units as will become more evident from the detailed description of the invention found herein below.